BA are pretty confident of where they stand, there has been no flight crew order forbiding the practice (or hadn't the last time we spoke about it) and and as far as my BA 747 mate is concerned it is still a legitimate course of action. As someone has already said if the FAA have a problem with the 747 flying around on 3 they should be talking to Boeing about re-writing the type certificate which seems to allow continued flight on 3 engines.